The present invention relates to a voltage regulator circuit for the field of smart card IC fabrication.
Integrated circuits with different voltage domains (external and internal voltages) normally contain voltage regulator circuits, for example as described on page 10 of the document xe2x80x9cLP2954/LP2954A 5V and Adjustable Micropower Low-Dropout Voltage Regulatorsxe2x80x9d from the National Semiconductor Corporation dated June 1999. The operating voltage for the integrated circuit is the potential difference between the ground connection (VSS) and the supply potential. This voltage, which is referred to in the following text as the internal supply voltage VDDint, is produced from an external supply voltage VDDext by using a complex voltage regulator circuit, which is formed by a series regulator. In order to simplify the explanation, the series regulator in the voltage regulator circuit specified according to the prior art is shown in FIG. 1 in a circuit configuration which illustrates the major method of operation of this circuit for driving a series regulation transistor.
The circuit that is illustrated in FIG. 1 has a series regulator L that has a series regulation transistor and that converts the external supply voltage VDDext to the internal supply voltage VDDint. An electrical potential, which is provided for controlling the voltage conversion process, is applied to the gate connection of the series regulation transistor M1, as the regulator terminal R of the series regulator. This signal is a regulator-internal analog signal. The internal supply voltage VDDint is provided for supplying voltage to the smart card IC IC. The core capacitor Ccore is also shown, in order to explain the operation of the circuit. The circuit of the series regulator is normally considerably more complex, and differs from the illustrated simple example whose functional principle is the same.
The voltage regulator circuit illustrated in FIG. 1 has the disadvantage that, when the external supply voltage VDDext is switched on quickly, overshoots occur in the internal supply voltage VDDint that are no longer tolerable beyond a specific integration level because they lead to damage to the thin gate oxide of the transistors that are connected to VDDint. FIG. 4 shows the voltage profiles of VDDint and VDDext, plotted against time, for the switching-on process.
In addition, the total number of only five connecting contact surfaces, which are normally referred to as pads, (VDDext, VSS, Clock, IO, Reset) and their switching-on sequence, in accordance with the ISO/GSM Standard, are also specified for smart card ICs. There is thus no possible way to achieve a desired transient response by providing additional pads or other switching-on sequences.
It is accordingly an object of the invention to provide a voltage regulator circuit for smart card ICs that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that is as simple as possible for smart card ICs, and in which the problem of overshooting during the switching-on process does not occur.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a voltage regulator circuit according to the invention with a series regulator that is known per se and is provided in order to produce an internal supply voltage VDDint from an external supply voltage VDDext. The voltage regulator circuit has a capacitor and a field-effect transistor connected in series between a connection of the external supply voltage VDDext and a regulator terminal of the series regulator. The field-effect transistor is provided as a transfer gate. An electrical potential, which varies in accordance with a control signal for a smart card IC, is provided for this transfer gate. In particular, the series regulator has a series regulation transistor that is configured with its source and drain terminals in series between the external supply voltage VDDext and the internal supply voltage VDDint, and whose gate connection is the regulator terminal of the series regulator, or is at least connected to the regulator terminal. The potential that is applied to the gate of the series regulation transistor can be regulated.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a voltage regulator circuit for smart card ICs, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.